UWB signal is defined as the signal that has the bandwidth to center frequency ratio greater than 0.25 or bandwith of 500 MHz or greater. In the past, UWB technology has found military applications in ground penetrating radars (GPR), wall penetrating radars, secure communications and precision positioning/trackings. However, nowadays there is also a growing interest in commercial use of UWB technology such as in Wireless Personal Area Networks (WPAN). FCC recently allocated the frequency range from 3.1 to 10.6 GHz for UWB communications.
This interest has been the result of increasing demand for much higher data rates on the order of hundreds of megabits, since future wireless networks requires very large transmission bandwidths to reach these data rates. Currently, most wireless data technologies such as Bluetooth, IEEE 802.11b have baseband signals up to tens of megabits, and the baseband signal is sent using an RF carrier, which is basically a narrowband communication technique.
There are mainly two alternative ways of UWB systems from the point of view of generating the UWB signal. One system is the so called the impulse radio, in which ultra narrow pulses with durations of picoseconds are generated and the generated time pulses may span a few GHz wide bandwidth. The other system is the multibanded approach so that multiple narrowband signals are generated independently and then combined to form the larger bandwidth of UWB signal.
For time domain impulses, there exist different waveforms that can be used for UWB signal such as Gaussian monopulse, Gaussian doublets, Rayleigh, Laplacian, wavelet monopulses, etc. Each of these waveforms can be designed for a specific center frequency and a required bandwidth. The frequency domain parameters of the pulses can be derived by the time domain parameters of the pulses, and vice versa.
Specifically, time duration of the pulse determines the frequency bandwidth occupied by the signal, the cycles per pulse. In other words, number of zero crossings determines the center frequency of the pulse and the pulse shape will provide the sidelobe levels of the signal as well as how the signal energy is distributed over the range of frequency bandwidth.
FIG. 1 shows the time domain waveform of an UWB monopulse for a duration of 400 picoseconds. This ideal Gaussian doublet is obtained as a derivative of a Gaussian waveform which is given by the following formula:
                              V          ⁡                      (            t            )                          =                              t            τ                    ⁢                      e                          -                                                (                                      t                    τ                                    )                                2                                                                        (        I        )            where τ (tau) is the time duration of the pulse chosen as τ=100 psec.
FIG. 2 shows the plot of the Gaussion monopulse waveform in frequency domain, wherein the center frequency of the Gaussion monopulse fc can be estimated as 2.7 GHz, which is determined by the number of zero crossings in total duration of the monopulse, τ. The 3-dB bandwidth is about 2.5 GHz, which is given as the inverse of the total duration of the pulse 1/τ. The envelope of the monopulse will determine how the total power of the monopulse will be distributed over the ultra wideband frequency bandwidth of the monopulse.
The UWB pulse is centered around a center frequency with an ultra wide bandwidth. In other words, the single pulse may have a very large bandwidth. However, if it is centered around the zero-frequency, the design of one compact antenna for all frequencies would be very difficult. This is due to the fact that for an efficient radiator at low frequencies, antenna size should be comparable to the wavelength, which is enormously large at low frequencies.
One way to obtain such kind of a pulse and eliminate the antenna size problem, is to first generate the zero-frequency centered pulse, then take the derivative of the pulse to eliminate the constant or DC part of the pulse and shift the center frequency from zero to a higher frequency fc.
Various designs of ultra wideband waveform generators are known in the art.
An example of such UWB waveform generator circuits is disclosed in BUCHEGGER, Thomas et al. “A Novel low-cost ultra wideband microstrip pulse forming network for gaussian monocycle generation.” 2003 International Workshop on Ultra Wideband Systems (IWUWBS). In this pulse forming network, microstrip lines with a short circuit termination was used to obtain the gaussian monocycle. However, with this approach there is a large ringing at the output.
Another example of such UWB waveform generator circuits is disclosed in HAN, Jeongwoo, et al. “A New ultra-wideband, ultra-short monocycle pulse generator with reduced ringing.” IEEE Microwave and Wireless Component Letters. June 2002, vol. 12, no. 6, p. 206-208. Han et. al use Step Recovery diode and short circuited transmission line to obtain the pulse and a simple RC filter to take the derivative. However, with this approach the pulse envelope properties are degraded which results in shape distortion in frequency domain and the amplitude of the pulse is lower due to the resistance loss.
Thus, there is a need for an UWB waveform generator circuit that can produce smooth and well-shaped pulses and achieve minimum losses in the resultant amplitude of the pulse and especially produce very small second order reflections also known as ringing. This is very important if the UWB waveform generator circuit is to be used in a receiver for determining a radar return typically a small amplitude signal.